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Description  |
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FIELD OF THE INVENTION
This invention relates generally to spectrographic method and apparatus
and, in particular, to an Atomic Force Microscope used with a source of
radiation selected to be absorbed by atoms or molecules to be
investigated. A cantilevered vibrating tip of the Atomic Force Microscope
is scanned over an illuminated sample surface and directly detects and
measures the resulting atomic or molecular increase of size, thereby
detecting both the presence and location of the atoms or molecules under
investigation.
BACKGROUND OF THE INVENTION
The term microscopy is employed where a surface is imaged with radiation of
a same energy. Where radiation of different or varying energies is used
the term spectroscopy is generally employed. Dual purpose instruments are
generally designated as microscopes even when they perform spectroscopic
investigation as well.
Spectroscopic analysis of surfaces at atomic scales is desirable for a
number of reasons, including the identification and characterization of
surface impurities in semiconductor, superconductive and other structures.
In U.S. Pat. No. 4,343,993, Aug. 10, 1982, Binnig et al. describe a vacuum
electron tunneling effect that is utilized to form a scanning tunneling
microscope. In an ultra-high vacuum at cryogenic temperature, a fine tip
is raster scanned across the surface of a conducting sample at a distance
of a few Angstroms. The vertical separation between the tip and sample
surface is automatically controlled so as to maintain constant a measured
variable which is proportional to the tunnel resistance, such as tunneling
current.
In a journal article entitled "Atomic Force Microscope", Physical Review
Letters, Vol. 56, No. 9, G. Binnig et al. at pages 930-933 described an
atomic force microscope that is said to combine the principles of the
scanning tunneling microscope and a stylus profilometer.
In U.S. Pat. No. 4,724,318, Feb. 9, 1988, Binnig describes an atomic force
microscope wherein a sharp point is brought near enough to the surface of
a sample to be investigated that forces occurring between the atoms at the
apex of the point and those at the surface cause a spring-like cantilever
to deflect. The cantilever forms one electrode of a tunneling microscope,
the other electrode being a sharp tip. The deflection of the cantilever
provokes a variation of the tunnel current, the variation being used to
generate a correction signal which can be employed to control the distance
between the point and the sample. In certain modes of operation, either
the sample or the cantilever may be excited to oscillate in a z-direction.
If the oscillation is at the resonance frequency of the cantilever, the
resolution is enhanced.
In U.S. Pat. No. 4,747,698 Wickramasinghe et al. describe a scanning
thermal profiler wherein a fine scanning tip is heated to a steady state
temperature at a location remote from the structure to be investigated.
Thereupon, the scanning tip is moved to a position proximate to, but
spaced from the structure. At the proximate position, the temperature
variation from the steady state temperature is detected. The scanning tip
is scanned across the surface structure with the temperature variation
maintained constant. Piezo electric drivers move the scanning tip both
transversely of, and parallel to, the surface structure. Feedback control
assures the proper transverse positioning of the scanning tip and voltages
thereby generated replicate the surface structure to be investigated.
In a journal article entitled "Atomic Force Microscope-Force Mapping and
Profiling on a Sub 100-A Scale", J. Appl. Phys. 61 (10), 15 May 1987, Y.
Martin et al., at pages 4623-4729 describe a technique for accurate
measurement of the force between a tip and a material, as a function of
the spacing between the tip and the material surface. The technique
features a tip that is vibrated at close proximity to a surface in
conjunction with optical heterodyne detection to accurately measure the
vibration of the tip. The technique enables the measurement of tip
displacements over large distances and over a wide range of frequencies,
which is a major advantage over the previous methods. The technique is
applicable to non-contact profiling of electronic components on scales
varying from tens of microns to a few tens of angstroms. A second
application is described wherein material sensing and surface profiling
are achieved simultaneously.
In a journal article entitled "High-resolution capacitance measurement and
potentiometry by force microscopy", Appl. Phys. Lett. 52 (13), Y. Martin,
D. W. Abraham and H. K. Wickramasinghe at pages 1103-1105 describe an
atomic force microscope employed for potentiometry and for imaging surface
dielectric properties through the detection of electrostatic forces.
The electron tunneling effect is shown to be applicable to spectroscopic
analysis is a journal article entitled "Tunneling Spectroscopy", B. J.
Nelissen and H. van Kemper, Journal of Molecular Structure, 173 (1988) at
page 141-156. This article describes the use of the Scanning Tunneling
Microscope as a spectroscopic probe. These authors note that spectroscopic
methods use energetic probes, usually photons, to gain desired
information. They further note that for spectroscopy in conducting solids
the use of photons is not an obvious choice, since the electrons inside
the solid can be used as spectroscopic probes.
In a journal article "Photothermal Modulation of the Gap Distance in
Scanning Tunneling Microscopy", Appl. Phys. Lett. 49 (3), 21 July 1986, by
Nabil M. Amer, Andrew Skumanich and Dean Ripple at pages 137-139 describe
the use of the photothermal effect to modulate the gap distance in a
tunneling microscope. In this approach, optical heating induces the
expansion and buckling of a laser-illuminated sample surface. The surface
displacement can be modulated over a wide frequency range, and the height
(typically .sctn. 1 Angstrom) can be varied by changing the illumination
intensity and modulation frequency. The method is said to provide an
alternative means for performing tunneling spectroscopy.
As is apparent the Scanning Tunneling Microscope (STM) and the Atomic Force
Microscope (AFM) have provided an efficient and accurate means to perform
the observation of atomic features on surfaces However, such prior art
techniques have not overcome the problem of providing an efficient and
accurate means to perform spectroscopy on the atomic and/or molecular
scale, although certain attempts have yielded some limited results, namely
voltage spectroscopy in STM, "peak force detection" spectroscopy with the
AFM, temperature spectroscopy with the Thermal Profiler, and Auger
spectroscopy with a Field emission microscope.
It is thus an object of the invention to provide apparatus and method for
performing spectroscopy at atomic scales.
It is another object of the invention to provide method and apparatus for
practicing Atomic Photo-Absorption Force Microscopy (APAFM) that
beneficially combines both atomic resolution and spectroscopy for use in
wide range of analytical applications.
SUMMARY OF THE INVENTION
The foregoing problems are overcome and the objects of the invention are
realized by an Atomic Photo-Absorption Force Microscope constructed and
operated in accordance with the invention. In accordance with the
invention a radiation source has a wavelength selected to be
preferentially absorbed by atoms or molecules associated with a sample
surface under investigation. Absorption of the radiation raises at least
one outer shell electron to a higher energy level, resulting in an
increase in radius of the atom or molecule. A tip coupled through a lever
to an Atomic Force Microscope is scanned over the surface and operates to
directly measure the resulting atomic or molecular increase of size,
thereby detecting both the presence and location of the atoms or molecules
under investigation. Operation in an a.c. mode by chopping the incident
radiation and measuring the corresponding a.c. induced tip movement
beneficially increases the sensitivity of the technique, particularly if
the a.c. frequency is chosen at a resonance of the tip-lever combination.
In accordance with a method of the invention there is disclosed a method of
performing spectroscopy at atomic scales. The method includes a step of
illuminating a sample with radiation having a characteristic wavelength
selected for being absorbed by atoms or molecules of interest such that at
least one outer shell electron is raised to a higher energy level,
resulting in an increase in a radius of the atom or molecule. The method
includes a further step of translating a probe tip proximal to the surface
of the sample, the probe tip being mounted such that it experiences a
detectable movement in response to being positioned near to an atom or
molecule of increased radius. The method also includes a step of detecting
the movement of the probe tip for indicating the presence of the atoms or
molecules of interest. The probe tip has at least two characteristic
resonant frequencies and the step of illuminating includes a step of
chopping the radiation at a frequency substantially equal to a first one
of the characteristic resonant frequencies. Furthermore, the step of
translating includes a step of oscillating the probe tip perpendicularly
to the surface at a second one of the characteristic resonant frequencies.
BRIEF DESCRIPTION OF THE DRAWING
The above set forth and other features of the invention will be made more
apparent in the ensuing Detailed Description of the Invention when read in
conjunction with the attached Drawing, wherein:
FIG. 1 is a block diagram, not drawn to scale, illustrating the APAFM of
the invention disposed relative to a sample; and
FIG. 2 is a diagram that illustrates APAFM tip displacement as a function
of atomic radius and illuminating radiation.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1 illustrates an APAFM 1 disposed relative to a sample. The APAFM 1
includes an Atomic Force Microscope (AFM) 10 that is similar in many
operational characteristics to the AFM described in the above mentioned
journal article entitled "Atomic Force Microscope-Force Mapping and
Profiling on a Sub 100-A Scale", J. Appl. Phys. 61 (10), 15 May 1987, Y.
Martin et al., at pages 4623-4729. The AFM 10 is configured to operate in
a repulsive force mode. A tungsten tip 12 disposed at the end of a wire
lever 14 is mounted on a piezoelectric transducer 16. The transducer 16 is
driven by a source 16a of alternating current (a.c.) and vibrates the tip
along a z-axis at the resonant frequency of the wire lever 14, which acts
as a cantilever. A laser heterodyne interferometer 18 accurately measures
the amplitude of the a.c. vibration. The tip/lever combination (12,14) is
also coupled to suitable piezoelectric transducers (not shown) for being
translated along an x axis and a y axis parallel to a surface 24a of a
sample 24.
In an illustrative embodiment of the invention the lever/tip (14,16) is a
unitary body comprised of a tungsten rod, etched into a cone, having a
length of approximately 460 microns, a base diameter of approximately 15
microns, and a final tip diameter of approximately 0.1 micron. The last 40
microns of the cone are bent at 90.degree.. The tip spring constant k, the
first and the second resonant frequency and the Q factor of the lever were
determined to be 7.5 N/m, 72 kHz, 200 kHz and 190, respectively.
It should be understood that these tip and other characteristics are
exemplary and are not to be construed in a limiting sense. Also, it should
be understood that while there is no intent to limit the scope of the
present invention by the theory presented below, this theory is believed
to be both accurate and consistent with observable facts and accepted
scientific principles.
The APAFM 1 further includes a radiation source 20 that provides periodic,
chopped radiation, indicated by the numeral 22, of selected wavelength to
illuminate the sample 24 disposed within the vicinity of the tip 12. The
radiation 22 is preferentially absorbed at the sample 24 surface 24a by
exciting electrons of atoms 26, or molecules, to a higher energy state 28.
One suitable radiation source is a focussed and chopped tunable laser,
such as a dye laser or a frequency doubled dye laser. The required
wavelength may vary from the near infrared, which is required to probe
molecular bonds, up to ultraviolet which is required to excite electrons
on low atomic orbitals. An acoustooptical-type modulator can be employed
to chop the radiation beam. Typically the chopping frequency is selected
to coincide with the lowest resonant frequency of the lever/tip (14,16),
or 72 kHz for the embodiment discussed herein. The absorption of the
radiation 22 results in an increase in radius of the atom or molecule from
a first radius (r1) to a second larger radius (r2).
In accordance with the invention the radiation source 20 has a wavelength
selected to be preferentially absorbed by atoms or molecules of interest.
The tip 12 is scanned parallel to and proximal to the surface 24a and
operates to directly measure the resulting atomic or molecular increase of
radius, thereby detecting both the presence and location of the atoms or
molecules under investigation. Operation in an a.c. mode by chopping the
incident radiation 22 and measuring the corresponding a.c. induced tip 12
movement has been found to beneficially increase the sensitivity of the
technique, particularly if the a.c. frequency is chosen at a resonance of
the tip-lever combination.
The tip/lever combination (12,14) resonates at two frequencies
.omega..sub.0 and .omega..sub.1 and the AFM 10 detects both of these
frequencies. From the vibration at .omega..sub.1, generated by the source
16a, the AFM 10 controls the spacing between tip 12 and sample 24 and
displays the surface topography. The radiation 22 is chopped at
.omega..sub.1 and the spectroscopy of surface 24a is derived from the tip
12 vibration at .omega..sub.1.
The invention advantageously exploits the characteristic of atomic
structure that causes the radius of an electronic orbital of an atom to
increase roughly with n.sup.2, where n is the principal quantum number of
the orbital. According to Slater's orbitals, the radius of an atomic
orbital has a value n.sup.2 /(Z-s) in units of Bohr radii, where Z is the
atomic number and (s) is some screening factor smaller than Z. It is
apparent from this formula that the size difference between two adjacent
orbitals at the periphery of an atom is of the order of one angstrom. In
the example of Rb+ which has 36 electrons, the approximate radii for the
orbitals 1s, 2s and 2p, 3s 3p and 3d, 4s and 4p are 0.1, 0.3, 0.9 and 3
Angstroms, respectively. The orbital of higher order (n=5) that
corresponds to an excited state of Rb+ exhibits a significantly larger
radius of approximately 6.0 Angstroms.
The "apparent" size of an atom or molecule is very similar to the size of
the external electronic orbital, as far as bonds with other atoms or
forces are concerned. Hence, exciting an atom by bringing an electron to
an orbital larger than the last normal orbital of the atom significantly
increase the apparent size of that atom, by up to several Angstroms.
The lifetime of an excited atom depends widely on a number of factors
including radiative or non-radiative decay to the fundamental state and
coupling to the surrounding media. For a radiative decay in the case of a
strongly allowed electric-dipole atomic transition in the optical
frequency, as is exploited in the APAFM 1, a value quoted by Siegman in
"An Introduction to Lasers and Masers", McGraw-Hill (1971) at page 100 is
10 nanoseconds (ns). However, atoms in a crystal or solid can exhibit more
rapid non-radiative decays, down to picoseconds (ps), due to a strong
coupling of the internal atomic oscillations to the surrounding crystal
lattice. However, even in this case a few transitions of selected atoms in
solids are so decoupled from lattice vibrations that they have relatively
long lifetimes. For example, Nd.sup.3+ in a has a 4 millisecond (ms)
lifetime [Siegman, p. 101-2].
Although it may seem apparent that the lifetime of the excited state is a
dominant and important factor for successful operation of the APAFM 1 such
may not be true for several reasons.
Firstly, the tip 12 of the AFM 10 does not measure an average size of the
atom, but the "peak" (r2) size of the atom, as shown in FIG. 2. In FIG. 2,
t1 is the average time interval between incident photons and t2 is the
life time of an excited atom or molecule. By example, with a maximum
radiation 22 flux of 1mW focused within a one micron spot, corresponding
to 10.sup.7 photons per second, the duty cycle (t2/t1) of the atom in the
excited state may be very small, as depicted by the narrow peaks in the
diagram of FIG. 2. Due to the strong repulsive forces between the tip 12
and the atom, and because the tip 12 cannot follow the fast transitions of
the atom, the tip is repelled from the atom to a distance 30 dictated by
radius (r2) of the excited state. The tip 12 maintains this distance 30
during the on-time of the incident radiation 22 and approaches the radius
r1 of the orbital of the ground state (distance 32) when the radiation 22
is off. The radiation chopping frequency is thus preferably tuned to a
resonance of the tip-lever (12,14) to increase sensitivity.
Secondly, a consideration of energy is even more appropriate in sensing
atomic size variations with the tip 12. The transitions that are
considered are transitions from a high order orbital to a non-populated
external orbital, i.e. Balmer (from n=2), Paschen (n=3) or Brackett (n=4)
transitions. The associated photon energy is typically a few electron
volts (eV). This energy of the incident photons is the energy that
eventually moves the tip 12. It can be shown that the energy required to
move the tip 12 is the spring energy:
##EQU1##
For k=100N/m,x=1 Angstrom and Q=200 this required spring energy is much
smaller than the energy of a single photon. Furthermore, several photons
are contributing to the tip 12 displacement during each "on" alternate of
the chopped radiation 22 frequency. Therefore, the presence of the tip 12
induces only a small perturbation to the atom that is being excited.
Bulk thermal expansion of the sample due to the radiation absorption may
induce some spurious tip 12 vibration at the tip-lever resonances. These
spurious vibrations are minimized by mounting the sample 24 on a
transparent holder 34 so that radiation absorption will occur only by
those atoms whose transition is tuned to the radiation wavelength.
Additionally, thermal effects may be suppressed by employing a second
radiation source 36 having a wavelength that differs from the wavelength
of source 20. The source 36 is chopped out of phase with the first
radiation source 20 and heats the bulk of the sample 24 so that its
temperature and thermal expansion remain substantially constant in time.
In practice the amplitude, chopping frequency and phase of the source 36
may be varied or adjusted to obtain a desired degree of cancellation of
spurious oscillation due to thermal effects.
For an electrically conductive sample, or a thin sample disposed upon an
electrically conducting substrate, spurious vibration may be cancelled by
employing an electrostatic force. The electrostatic force is generated by
applying an a.c. voltage between tip 12 and sample 24 in a manner
disclosed in the aforementioned journal article entitled "High-resolution
capacitance measurement and potentiometry by force microscopy", Appl.
Phys. Lett. 52 (13), Y. Martin, D. W. Abraham, H. K. Wickramasinghe.
Preferably the a.c. signal has a frequency that is approximately one half
of the lower resonant frequency, or 36 kHz for this embodiment, indicated
by the source 38 designated by .omega..sub.1/2.
The energy of the electronic transitions actually detected may differ from
the orbital energies tabulated by using classical methods. That is, in the
APAFM 1 the atoms being illuminated are located at the surface 24a of the
solid sample 24, as opposed to atoms within the bulk of a material as are
typically considered by classical analytic technique. Thus, surface
effects may influence the energy levels of the atoms. The presence of the
tip 12 may also alter the energy levels, specially if the tip 12 is
electrically biased. However, these factors need not be considered in a
negative or limiting sense in that they allow new types of spectroscopy to
be done. That is, atomic scale spectroscopy is accomplished that addresses
surface atoms and that operates in accordance with functions of local
parameters such as electric field force and surface effects.
The sensitivity of the APAFM 1, based on considerations of radiation flux
and of photon and tip energies, is suitable for use with a number of
materials including insulators, where tunneling and inverse photo-emission
spectroscopy are not feasible.
Thus, while the invention has been particularly shown and described with
respect to a preferred embodiment thereof, it will be understood by those
skilled in the art that changes in form and details may be made therein
without departing from the scope and spirit of the invention.
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